Recent years have seen the development of automobiles in which an auxiliary power supply is installed in addition to the main battery in order to improve fuel efficiency and give consideration to environmental issues. For example, there are cases where regenerative energy obtained during braking is stored in the auxiliary power supply and then used when supplying power to electrical components during traveling, thus reducing the amount of power generated by the alternator and improving fuel efficiency. There are also cases where power is supplied from the auxiliary power supply to the starter when restarting the engine after idling stop, thus preventing power flickering in electrical components and main battery degradation caused by a sudden drop in voltage in the main battery.
An apparatus is disposed between the main battery and the auxiliary power supply in order to switch the power supply mode according to the operating state of the vehicle, such as traveling or idling stop. Mechanical relays that conduct a large current have conventionally been used in this type of apparatus, but replacement with semiconductor switching elements has been proposed in order to achieve size reduction, lifetime extension, and noise reduction. A power semiconductor such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is used as the semiconductor switching elements (see JP 2009-146933A).
Also, paths for the conduction of a large current need to be connected reliably, and therefore instead of connecting connectors to each other, connection has been performed by overlaying terminals at the ends of paths and fastening them together using bolts and nuts. In this case, terminals can be connected by inserting the shaft portion of a bolt into the through-holes of the terminals, bringing one of the terminals into contact with the seat surface, and fastening a nut to the other side.
In this case, the bolt is generally molded using a die, and it is generally the case that a right angle is not completely formed at the location where the seat surface and the outer circumferential surface of the shaft portion intersect, and instead a curved surface bulges in the boundary portion between the seat surface and the outer circumferential surface of the shaft portion. The through-holes of the terminals are given a small hole diameter in order to ensure a sufficient area of contact between the terminals, and therefore if a curved surface is formed at the boundary portion between the seat surface and the outer circumferential surface of the shaft portion, there is concern that the hole edge of the through-hole of the terminal will become caught on the curved surface such that the terminal will not be connected at a predetermined position, thus increasing contact resistance between terminals.
The present invention was achieved in light of the above-described situation, and an object thereof is to suppress an increase in contact resistance between terminal portions.